Spectrometric analysis is a non-invasive and non-destructive manner in which to determine both qualitative and quantitative properties of compositions. Infrared analysis and more particularly near-infrared ("NIR") analysis, is particularly suited to the analysis of organic compounds. The infrared absorption spectrum is highly characteristic, and is sometimes referred to as the molecular fingerprint. The natural vibrational frequencies of molecules and crystals fall within the infrared range and therefore the infrared region is valuable for the study of the structure of matter. Certain molecular bonds are prone to vibrate when exposed to characteristic wavelengths of infrared light which causes the molecules to absorb infrared light. Near infrared spectroscopy takes advantage of this activity by measuring the absorption of an unknown sample at various wavelengths throughout the near infrared range. Infrared light which is either reflected from or transmitted through a sample exhibits a highly characteristic spectrum showing the absorption of the sample at various predetermined wavelengths. The wavelength and magnitude of the absorption, as revealed in a spectrograph (a graphical representation of the absorbance values), can be used to determine information about the molecular structure and composition of the sample. Infrared spectrometry has proven to be a valuable tool for analysis of a wide variety of products including milk, grains, oils, gasoline, alcohols, and pharmaceutical products.
It is often desirable to use an optical probe to interface with the sample by being directly inserted into a sample gas or liquid. In general, measuring devices used for infrared spectroscopy require a near infrared light source and a light detector contained in an instrument known as a spectrometer. Light which has been either reflected from or transmitted through a sample is broken down into narrow wavelength bands either before or after interaction with the sample. In some arrangements, fiber optic cables transmit the light to and from the sample in a probe which provides an appropriate interface with the sample. The narrow wavelength bands are then directed to a light detector which then transmits a signal indicative of the intensity of the detected light. The signal is then analyzed or interpreted to yield absorbance data which in turn provides information about the constituent make-up of the sample.
Absorbance measurements are generally of reflectance, transmittance or combined transmittance and reflectance. Reflectance measurements involve directing light at a sample and then collecting the light which is diffusely reflected or scattered either from the surface of a sample or from molecules or crystals contained within a sample. A portion of the light diffusely reflected from the sample is directed back to a light sensitive detector system where it is converted to a signal. In the detection operation, the output of the photodetectors is sampled to yield values which indicate the intensity of the reflected light at the narrow wavelength bands. The analysis of reflectance measurements involves scanning a standard, often in the form of a white reflective tile. The value of the signal generated from light reflected from the standard is compared with the light reflected from the sample to yield a value representing the absorbance of the sample. Reflectance measurements are routinely employed in the measurement of solids and non-Newtonian matter such as chemical powders and solid agricultural products.
A second type of infrared analysis, which is referred to as transmittance or transmission, involves directing infrared light at a sample and then measuring the unscattered light which has passed through the sample. Incident light which has passed directly through the sample without scattering is directed to a detector. The detector then generates a signal from which absorbance values of the substance being analyzed are determined. As in the case of reflectance measurements, an absorption spectrum can be created which sets forth the absorbance of the sample plotted as a function of wavelength. Transmittance measurements also require a standard or reference measurement which approximates 100 percent transmittal of light. Usually the reference measurement is taken with an empty sample cell or a cell containing a clear liquid. The value of the signal from light transmitted through the sample is compared with the value of the signal obtained from the standard to yield an absorbance value. Because of the limits in the sensitivity of the instruments, transmission absorption spectra are generally limited to samples which are relatively transparent to infrared light.
A third, hybrid method of spectrometric measurement involves the simultaneous collection of both light diffusely reflected from and light transmitted through a sample. This measurement also involves providing an illuminating source which transmits infrared light to a probe immersed within the sample. Sample material can flow into a slot provided on the end of the probe and defined by a window and an opposite mirror. Light is transmitted from the probe through the window and then through the sample, where it impinges on the mirror. From the mirror, light is reflected back again through the sample and back through the window, where it is collected by suitable means, such as fiber optics, and directed to a detector. In this arrangement, some of the light from the illuminating source falls directly on matter suspended within the sample and consequently is scattered by the sample. A portion of the scattered light is reflected back to the collection optical fibers. Thus, in certain circumstances, the collection fibers capture both unscattered light which has been transmitted through the sample and light which has been diffusely reflected by the sample.
Most batch reaction processes begin as translucent media and, as the reaction proceeds, the media can become strongly light scattering. Measurement of this type of reaction process presents significant challenges with conventional spectroscopic techniques. For example, to successfully monitor initial reaction conditions, a transmittance measurement is necessary. To provide adequate measurement as light scattering increases, a reflectance measurement is necessary. Therefore, with traditional spectroscopic instrumentation, two measurement modes are necessary usually requiring two probes for entry into a fluid being processed.
Most instruments are designed to collect either the reflected light or the transmitted light from a sample. This allows for the simplest designs and optics but is not optimum for the application of the technology. It is estimated that 80% to 90% of processing is batch processing rather than continuous processing. Most reactions start as mixtures of slurries, or as liquids going to solids, or as solids going to liquids. In these cases, the current analysis systems allow only a portion of the reaction to be monitored.